Coating iron or steel with aluminum or an alloy thereof



Patented Dec. 7, 1937 r V FF or COATKNG IRQN R STEEL WITH ALUMINUM OR ANALLOY THEREOF Ernst Johan Mauritz Mattsson, Stockholm. Sweden, assignorto Frans Nilsson, Stockholm,

Sweden No Drawing. Application February 28, 1935. Se-

rial No. 1934 8,722. In Great Britain March 5,

s v e I i Claims. (Cl. 91-102) The invention relates to a method ofcoating or alloying with aluminium or an aluminium alloy thesurfaceof anartlcleof iron orsteel by-immersionfof the article in a bath of moltenaluminium or aluminiumalloy.

It isknown that the surface of articles to be treated in thismanner-must be clean and free from impurities during the treatment.

In order to ensure this cleanliness the following procedure has beenproposed: The article to be treated is cleaned by mechanical meansand/or corroded in a dilute acid and/or subjected to a treatment-bywhich volatile products are formed which when heated volatilize, therebyremoving the impurities. The article is thereupon immersed in a moltenflux consisting of CaClz, fi'aCl, KCI; Na3AlFs and ZnClz. The articleheated in this flux is then removed to the molten aluminium bath. a r

In this procedure dlfliculties have arisen. First, it has been foundnecessary to immerse the article in the flux bath immediately after themechanical cleaning or corroding operation, because the least quantityof oxide prevents the 'adhesion of the coating to the oxidized surface.Secondly, even if the utmost attention has been given to the thoroughremoval of oil, oxides or the like before immersion in the flux bath,the procedure has often failed to procure the necessary adhesion of thecoating.

According to the present invention, in order to obtain a good coatingthe article to be treated is first immersed in a flux bath containing amolten mixture, of alkali or other fluorides or fluorine compounds.suchas NaaAlFs, and alkali salts such as alkali chlorides, and optionallyother salts, preferably chlorides such as ZnClz, but not containing anycompound of calcium; the article is kept in this bath until oxidesand'the like have been removed and the article has been properly heated,after which the article is removed and immersed in the bath of moltenaluminium .or aluminium containing up to 5 per cent of other metalingredients.

It is essential that the flux bath should contain no calcium compound,since calcium is apt to form insoluble compounds such as calcium carbidewhich prevent the proper adhesion of the aluminium to the metal. It'isrecommended that a substantial amount of fluoride (more than 5 per cent.of NaaAlFt) should be present in the bath. If this be the case and nocalcium compound is present the fiux will quickly remove rust and otheroxides from the surface of the article to be treated. A pre-treatmentfor the removal of rust or the like as prescribed in the earlier art isthus made superfluous. Of course, if the articles to be treated arecoated by a thick layer of oxides as a result of prolonged heatingduring the manufacture (as by casting) it may be serviceable first toremove this layer in any known manner in order to shortenthe treatmentin the flux bath, but once this is done the articles may be allowed tocorrode and become rusty without this being any serious obstacle toobtaining a faultless coating of aluminium or aluminium alloy.

It has further been found that if potassium chloride is used along withsodium chloride the proportion of potassium chloride should not be toogreat. Preferably the proportion of the salts to each other should beaccording to their equivalent weights viz. about 40 parts by weightpotassium chloride to 30 parts of sodium chloride. Other proportions,however, maybe used, but when too great a quantity of potassium chlorideis present the flux is apt to adhere to the article treated when thelatter is transferred to the metal bath, instead of rising properly tothe surface of the bath and leaving the article well cleaned andsuitable iorbecoming coated with aluminium or its alloy.

Otherwise the proportions andnumber of ingredients of the flux bath mayvary within wide limits; The temperature of the flux bath depends uponthat of the metal bath which again may vary according to the kind ofarticle to be treated and the kind of result to be attained, as known tothose skilled in the art. The temperature of the flux bath should not bemuch lower and preferably a little higher than that of the metal bath.

The composition of the flux bath, therefore, will vary according tocircumstances. As an example of a flux which has been found useful forseveral purposes is given below:

Per cent by weight NaaAiFe 2.11012 (dehydrated) 2o NaCl 3o KCl 40 It isto be understood that this composition of the flux bath is given as anexample only.

What I claim is:--'

l. A method of aluminizing surfaces of ferrous metal bodies, whichcomprises cleaning and heating the body by immersing the body in amolten flux bath of the salt type containing about 10 per cent by weightof sodium alumirium fluoride,

about per cent by weight of sodium chloride, about per cent by weight oipotassium chlo ride and about 20 per cent by weight of zinc chloride butfree from any salt of calcium, removing the heated body from said bathand immers; ing same while in heated state in a molten bath containingaluminium.

2. A method of aluminizing the surface of a ferrous metal body whichcomprises cleaning and heating the body by immersing the same in amolten flux bath of the salt type the active ingredients of whichconsists oi alkali metal fluoride and chloride selected from thechlorides of one or more of the metals oi the group consisting of thealkali metals and zinc but free from any salt of calcium, removing thebody from said bath and immersing the body in a molten bath containingaluminum.

3. A method'oi aluminizing the surface of a ferrous metal body whichcomprises cleaning and heating the body by immersing the same in amolten flux bath of the salt type the active ingredients of whichconsist of alkali metal fluoride and chloride selected from thechlorides of one or more 0! the metals of the group consisting of thealkali metals and zinc but free from any salt of calcium, removing thebody from said bath and immersing the body in a molten bath containingaluminum, the amount of alkali metal fluoride being more than live percent.

4. A method of aluminizing the surface oi. a ferrous metal body whichcomprises cleaning and heating the body by immersing the same in amolten flux bath of the salt type the active ingredients of whichconsist of alkali metal fluoride and chloride selected from thechlorides of one or more of the metals of the group consisting of thealkali metals and zinc but free from any salt oi calcium, removing thebody from said bath and immersing the body in a molten aluminum bathcontaining up to live per cent of metal ingredients. I

5. A method of aluminizing surfaces of ferrous metal bodies whichcomprises cleaning and heating the body by immersing the remain a moltenflux bath of the salt type consisting of sodium aluminum fluoride,sodium chloride, potassium chloride and zinc chloride but free from anysalt oi calcium and in which bath the sodium aluminum fluoride is morethan i'ive per cent and the proportions of the sodium chloride and thepotassium chloride are substantially in accordance with their equivalentweights, removing the heated body from said bath and immersing the samewhile in a heated state in a molten bath containing aluminum.

' ERNST JOHAQI MAURITZ MATTSSQN.

